Ground-State Photoneutron Reactions in Carbon -14 and OXYGEN-18.

Differential photoneutron cross sections were measured by time-of-flight techniques for the reactions ^{14}{rm C}(gamma,n _0)^{13}{rm C} and ^{18}{rm O}( gamma,n_0)^{17}O as functions of laboratory angle between 48^ circ and 138^circ over the excitation energy region from 10 to 26 MeV. Angular distribution coefficients for the ^{14}{rm C}(gamma,n _0) reaction were extracted as functions of excitation energy between 10 and 22 MeV while angle -integrated cross sections were obtained between 10 and 26 MeV. The angular distribution results support a mechanism of dominant E1 absorption in the energy region from 13 to 22 MeV where an average value of a _2 = -0.5 indicates p _sp{1over2}{-1}d_ {3over2} single-particle neutron transitions. Angular distribution information suggests that much of a prominent resonance at 11.3 MeV (with an integrated cross section of about 1.03 MeV-mb) is due to an M1 transition from the ground state of ^{14}C. If this is the case, there is little fragmentation of the M1 strength in ^{12}C brought about by the presence of valence neutrons. When combined with the observation of the lack of a structured pygmy E1 resonance below the GDR region, these results suggest that a model of ^ {14}C as a ^{12} C "core" with two weakly coupled valence neutrons is not totally appropriate. The angle-integrated cross section for the ^{18}O(gamma,n_0 ) reaction was extracted between 14 and 26 MeV and angular distribution information was obtained from 14 to 20 MeV. The present data indicates that between 14.5 and 20 MeV, the ground state channel contributes only about 25% of the total transition strength found in the neutron channel. The integrated cross section between 14.5 and 20 MeV is 12 +/- 1 MeV-mb corresponding to about 4.5 +/- 0.4% of the TRK sum rule value for ^{18}O. No evidence is observed for the predicted T_ {<} strength near 25 MeV. The present T_{<} cross section between 20 and 26 MeV represents about 10% of the total photoneutron cross section. The present data support the application of the weak coupling model in describing the ^ {18}O nucleus. The isospin splitting energy and the sum-rule values derived from the present analysis are in agreement with theoretical predictions. Also presented in this thesis is a systematic comparison of the ground-state photoneutron cross sections of the naturally-occurring carbon and oxygen isotopes. (Abstract shortened with permission of author.).